This invention relates to the apparatus and method for the measurement of neutron moderating and absorbing properties of objects and more particularly to an apparatus which does not require transmission through the object being measured.
There is a need for the determination of the quantity of hydrogen in objects and also for the determination of the amount of neutron absorbers, such as boron, in nuclear fuel racks submerged under water. Presently such measurements are made by apparatus which relies on transmission through the object. Such an apparatus and method is schematically illustrated in FIG. 1. A source of fast neutrons 11 provides neutrons having energies in the range of 100-5,000 KEV. Such neutron sources are available from isotopic sources such as the spontaneous fission source, Cf.sup.252, or alpha-neutron sources such as Pu.sup.238 Be. The high energy neutrons impinge upon the moderator 12 which thermalizes some of the neutrons into lower energy neutrons. The fast neutrons may, for example, be moderated by polyethylene material. Some of these thermal neutrons pass through the thermal neutron absorbing sample or object 13 and are detected by thermal neutron detector 14. The detector may be, for example, a B.sup.10 F.sub.3 detector or a large H.sup.3 detector. The detector 14 detects only thermalized neutrons and does not react or respond to fast neutrons. The output of the detector is applied to a counting or indicating means 15. A count is obtained in the absence of the object 13 so that the detector 14 counts the neutrons moderated in the moderator 12. The sample or object 13 is then interposed between the moderator 12 and detector 14 and a new count is obtained. The difference in counts is a measure of the absorbing or moderating properties of the object 13.
There are, however, many applications where it is not possible to place the sample between the moderator 12 and the detector 14. For example, there is need for a method and apparatus for the determination of hydrogen content of such materials as concrete and wood in the construction industry. It would be virtually impossible to place the detector on the other side of a concrete slab which is laid upon the ground.
Another example is the determination of the water content of the human lung. Such a determination would provide means for detecting the condition known as "pulmonary edema". However, most measurements presently performed for the detection of pulmonary edema are transmission measurements. It is relatively difficult to differentiate the water content of the lung from the other fluid content of the body both in front and in back of the lung. If a measurement could be made from the front of the body a large part of the interfering fluid content would be eliminated. There is also a need for the verification of the neutron absorbing properties of nuclear fuel storage racks during a period when the racks are immersed in water. Again, it is very desirable that this measurement be accomplished from one side, as for example, with a probe positioned in a location where the fuel is to be inserted.
As used herein, the term "moderator" refers to atomic species such as berylium, carbon and particularly hydrogen which reduced the energy of neutrons by elastic collisions. The term "inelastic scatter" refers to material such as iron, nickel or tungsten which principally reduced neutron energy by the discrete inelastic process of absorption and re-emission at lower energy. The terms "absorber" or "poison" refer to materials such as boron, cadmium or gadolinium which absorb thermal neutrons. In this connotation, thick sections of less absorbent material such as steel are considered poisons. The term "black" refers to an absorber which absorbs substantially all of the incident thermal neutrons. The term "neutron source" as used in this application refers either to an isotopic source of fast neutrons such as Pu-Be, PuLi, Cf.sup.252, or to an accelerator neutron source such as a Van DeGraaf generator. The term "neutron detector" refers to a detector sensitive to thermal neutrons including such detectors as BF.sub.3 or He.sup.3 proportional counters and thermal neutron sensitive scintillation detectors. The term "fuel" or "nuclear fuel" as used herein refers to fresh or spent nuclear fuel elements containing sufficient fissionable material so that their storage requires special attention to criticality limits. The term "fuel storage racks" refers to special racks containing poison materials such as boron. The term "dry assay" refers to verification of the poison content of racks not flooded with water while the term "wet assay" refers to the assay after the racks are immersed in water. The individual openings in a fuel rack, accommodating one fuel element are described by the term "cell" and normally there are many cells in each fuel rack.